The present invention relates to filtering and, more particularly to filtering images.
The fine structure (texture) of ultrasound pulse-echo images has a mottled or granular appearance which has been called speckle because of its similarity to the equivalent optical phenomenon, laser speckle. The phenomenon of speckle results directly from the use of a coherent radiation for imaging and occurs when structure in the object (in this case the body), on a scale too small to be resolved by the imaging system, causes interference to occur between different parts of the wave received from the object region corresponding to a given point in the image. The term "speckle" as used in this specification is intended to include speckle in ultrasound pulse-echo images as well as similar phenomena in images obtained using other coherent radiation sources.
When the ultrasonic scatters are random and finely distributed the speckle generated is known as "fully developed speckle". It has a mean and modal value determined by the strength of the scattering but all other properties are independent of the object structure and are characteristic of the instrument used to produce the image. Both the amplitude (grey level) probability distribution and the average speckle cell will have a shape determined by the instrument, which may vary within the image.
It is now agreed by many authors that it would be desirable to remove, or reduce, the speckle in pulse-echo images since its presence degrades the apparent resolution in the image to a point well below the diffraction predicted value and it interferes with the visual assessment of small differences in mean grey level or texture. A number of methods have been suggested for achieving this, most of the methods involving the averaging or multiple images of the same object structure, varying one or more instrument parameters so that the speckle patterns in the images are significantly decorrelated. This reduces the variance of the speckle modulation and thus makes the mean level and object texture easier to perceive. Additionally it is possible to reduce the variance of the speckle by smoothing the image with two-dimensional low pass filtering. A simple filter (even one which varies spatially to account for the spatial variation of the speckle characteristics) will, however, also blur the object related information which one would wish to preserve. To date, post-formation image filtering methods have not been successfully applied to remove speckle from ultrasonic pulse-echo images.
What is described herein is to be distinguished from two previous publications. Firstly, the specific embodiment described to illustrate the utility of the method resembles a multiplicative noise filter developed by Lee ("Speckle analysis and smoothing of synthetic radar images" Computer Graphics and Image Processings 17 24-25 (1981)) to smooth synthetic aperture radar images, which also suffer from speckle. The differences are that the example given above is intended to only illustrate a general class of "intelligent" filters based on a multivariate description of speckle and they are not limited simply to filtering multiplicative noise. Furthermore, there are some specific differences in the definition of the value of the constant, k, and in the manner of derivation of the filter.
Secondly, Dickinson ("Reduction of speckle and ultrasound B-scans by digital processing" Acoustical Images 213-224 (Plenum Press)) has attempted to smooth speckle in ultrasound pulse-echo images using an unsharp masking filter where the coefficient k is controlled by the local mean, x, only. This attempt, which was designed to preserve large amplitude echoes from discrete structures such as blood vessels appeared to be relatively unsuccessful and certainly would not have been able to cope with two different texture regions which possessed the same mean level. The local means is not a parameter which can be used to recognise fully developed speckle. It is in fact the only speckle characteristic which is determined by the object. Thus any resemblence to the work of Dickinson is entirely superficial since the class of filters described here have quite a different derivation and a different application to that which he described.